Cartesian polarizers utilizing photo-aligned liquid crystals

Abstract

Cartesian polarizers utilizing liquid crystal polymers (LCPs) are provided for use as laser protection filters in optical pick-up units. The LCPs are photo-aligned and cross-linked into a polymer host to provide a durable filter. According to one embodiment, the LCP molecules exhibit a cholesteric phase and are coupled to one or more quarter-wave retarders. According to another embodiment, the LCP molecules are used to form a multi-layer stack using the giant-birefringence optics effect.

1. A method of fabricating a Cartesian polarizer comprising the steps of:
 providing at least three birefringent elements layered on a transparent substrate, each birefringent element including:
 a polymeric photo-aligned alignment layer; and 
 a cross-linked liquid crystal polymer layer, 
 
 wherein the optical thickness of each of the three birefringent elements is selected such that the Cartesian polarizer transmits light at a predetermined wavelength having a first polarization, and reflects light at the predetermined wavelength having a second orthogonal polarization, the light transmitted at the predetermined wavelength emitted from a laser diode in an optical pick-up unit, and 
 wherein the Cartesian polarizer is a giant-birefringence optics polarizer. 
 
     
     
       2. A method according to  claim 1 , wherein the step of providing each of the at least three birefringent elements includes:
 applying a solution of a linear photopolymer on a surface to provide a polymeric alignment layer; 
 irradiating the polymeric alignment layer with polarized UV radiation to induce a predetermined structure and form the polymeric photo-aligned alignment layer; 
 applying a first solution of liquid crystal polymer precursor to the polymeric photo- aligned alignment layer to form a liquid crystal polymer layer, and 
 irradiating the liquid crystal polymer precursor layer with UV light to provide the cross-linked liquid crystal polymer layer. 
 
     
     
       3. A method according to  claim 1 , wherein the at least three birefringent elements include a first plurality of A-plate elements alternating with a second plurality of C- plate elements, and wherein adjacent elements in the first and second plurality of elements have a combined optical thickness substantially equal to a half-wave at the predetermined wavelength. 
     
     
       4. A method according to  claim 1 , wherein the at least three birefringent elements include a first plurality of O-plate elements alternating with a second plurality of O- plate elements, and wherein adjacent elements in the first and second plurality of O-plate elements have a combined optical thickness substantially equal to a half-wave at the predetermined wavelength. 
     
     
       5. A method according to  claim 4 , wherein the first and second plurality of O-plate elements have their optic axes in the same plane, and wherein the optic axes of the first plurality of O-plate elements has a tilt angle that is smaller than the tilt angle of the optic axes of the second plurality of O-plate elements. 
     
     
       6. A method according to  claim 4 , wherein the first and second plurality of O-plate elements have their optic axes in orthogonal planes. 
     
     
       7. A method according to  claim 4 , wherein the at least three birefringent elements include a first plurality of A-plate elements, wherein the first plurality of A-plate elements alternates with a second plurality of substantially isotropic elements, and wherein adjacent elements in the first and second plurality of elements have a combined optical thickness substantially equal to a half-wave at the predetermined wavelength. 
     
     
       8. A method according to  claim 1 , wherein the at least three birefringent elements include a first plurality of O-plate elements, wherein the first plurality of O-plate elements alternates with a second plurality of substantially isotropic elements, and wherein adjacent elements in the first and second plurality of elements have a combined optical thickness substantially equal to a half-wave at the predetermined wavelength. 
     
     
       9. A method according to  claim 1 , wherein each polymeric photo-aligned alignment layer and each cross-linked liquid crystal polymer layer has an optical thickness substantially equal to one quarter-wave at the predetermined wavelength such that the at least three birefringent elements provide a quarter-wave stack of alternating layers. 
     
     
       10. A method according to  claim 9 , wherein each cross-linked liquid crystal polymer layer is one of an A-plate and an O-plate. 
     
     
       11. A method according to  claim 10 , wherein each polymeric photo-aligned alignment layer is substantially isotropic. 
     
     
       12. A method according to  claim 1 , including a first anti-reflection coating deposited on a second surface of the transparent substrate. 
     
     
       13. A method according to  claim 12 , including a second anti-reflection coating deposited on surface of the at least three birefringent elements. 
     
     
       14. A method according to  claim 12 , including a second transparent substrate, the second transparent substrate having a first side laminated to the at least three birefringent elements and a second side having a second anti-reflection coating deposited thereon. 
     
     
       15. A method according to  claim 1 , including a hard coat layer for protecting the at least three birefringent layers.